The metabolism of human procollagen is being studied with respect to: 1) the chemical structure of the nonhelical carboxy terminal sequences of procollagen; and 2) the number and modes of action of enzymes responsible for the conversion of procollagen to collagen. Antibodies to the carboxy terminal sequences of procollagen are being employed in electronmicroscopic and immunofluorescent studies of collagen fibirllogenesis. The structure and metabolism of procollagen is being studied in cultured fibroblasts from patients with inherited diseases of connective tissue. A radioimmunoassay for procollagen is being applied to sera and other body fluids of patients with diseases affecting connective tissues.